Magnesium oxide films are utilized as dielectric protective layers for AC type PDPs (altered current type plasma display panels), insulating films for MRAMs (magnetic resistance memories) and TMR elements (tunnel magnetic resistance elements), moisture adsorbing layers for organic EL elements and barrier films, and the like.
These magnesium oxide films are generally produced by deposition processes such as an electron beam deposition process, a spatter process and a CVD process. However, the deposition processes require vacuum apparatuses such as a vacuum chamber and a vacuum pump, and thus the production equipment becomes large. Therefore, a method including applying a dispersion liquid of fine magnesium oxide particles by an application process and drying the dispersion liquid has been investigated in recent years, and it is already known that an alcohol dispersion liquid of fine magnesium oxide particles is preferable for the application process and is advantageous for producing a thin film since any vacuum apparatus is not required and the film formation velocity is fast.
As the alcohol dispersion liquid of fine magnesium oxide particles, for example, Patent Literature 1 describes a fine magnesium oxide particle dispersion liquid containing a monovalent alcohol having 3 to 5 carbon atoms, and fine magnesium oxide particles that are dispersed in the range of from 0.05 to 20% by mass in the monovalent alcohol, wherein the particle dispersion liquid has D50 of the fine magnesium oxide particles measured by a dynamic light scattering method in the range of from 5 to 100 nm. Furthermore, Patent Literature 2 describes that the fine magnesium oxide particle dispersion liquid described in Patent Literature 1 is preferably used for forming a dielectric protective layer for an AC type PDP. These documents describe that, as a method for producing a fine magnesium oxide particle dispersion liquid, fine magnesium oxide particles having an average primary particle diameter within the range of from 5 to 100 nm and a monovalent alcohol having 3 to 5 carbon atoms are mixed, and the mixture (a dispersion liquid) is then subjected to a dispersion treatment by a pulverizing apparatus using beads having an average particle diameter of from 20 to 300 μm.
Furthermore, for example, Patent Literature 3 describes a fine magnesium oxide particle dispersion liquid containing a polar organic solvent, and fine magnesium oxide particles that are dispersed in an amount in the range of from 0.05 to 20% by mass with respect to the total amount of the dispersion liquid, wherein the particle dispersion liquid has D50 of the fine magnesium oxide particles measured by a dynamic light scattering method in the range of from 5 to 20 nm, and D10/D90 of 0.3 or more. This document describes, as a method for producing the fine magnesium oxide particle dispersion liquid, a method including a step of preparing a dispersion liquid by dispersing a magnesium oxide powder formed of fine magnesium oxide particles having an average primary particle diameter in the range of from 5 to 20 nm in a polar organic solvent to thereby prepare a dispersion liquid, and a step of forming a dispersion liquid of fine particles by applying a pressure to the dispersion liquid to thereby cause a dispersion liquid jet flow, then branching the dispersion liquid jet flow into two or more dispersion liquid jet flows, and allowing the respective dispersion liquid jet flows to collide under opposing to thereby break the magnesium oxide powder in the dispersion liquid to give a dispersion liquid of fine particles. Furthermore, specifically, a dispersion liquid in which a magnesium oxide powder had been put into butyl alcohol was produced in Examples.
In addition, Patent Literature 4 describes a metal oxide particle dispersion liquid containing: metal oxide particles having, in a particle diameter measurement by a dynamic light scattering method, an average particle diameter D50 in the range of from 15 nm or more and 40 nm or less, and D95 of 100 nm or less, and a coefficient of variation (CV value) of particle diameters in the range of 30% or more and 60% or less; a dispersant; and a dispersion medium, and the metal oxide particle dispersion liquid was produced in Examples by adding titania particles to a dispersion medium containing a dispersant and a dispersant aid under stirring with a homogenizer to give a slurry, and finally treating with zirconia beads.